Of course they say so, but it seems in Ghost Story/Changes Mab has some power over the cold and dark of the deeps...
Well, to be fair:
a/ "cold" and "dark" are pretty Queen-of-Winter sorts of things, after all!
b/ that scene at the end of Changes was the culmination of Mab's gambit to gain her new Knight. I expect she handled it personally... and likely she had some of her other heavy hitters on-hand in case any other powers wanted to intervene.
c/ we've seen that Demonreach has scope as far as the edges of the lake.
So, as Harry was aboard the Water Beetle, considering his upcoming date with Murphy: Mab (and likely other Winter heavyweights) were there, and maybe even Demonreach.
And I don't think the Fomor had any
motivation to be active at that specific harbor, on that specific day; I don't think they (and/or their allies) were tracking Harry very closely, at that point in time.
I don't think she has the kind of sovereignty under water that she has above. One of her titles is "Queen of Air and Darkness," which pretty explicitly leaves out the underwater realm. But she's still terribly-powerful, and entirely capable of conducting a brief mission outside the strict "Winter" domain, particularly in (winter-friendly) "cold and dark."
But... does she have the kind of power needed for
sustained surveillance under the water, with Corb & the other Fomor against her, and even Ethniu? To spot patterns? Does she know all the underseas Ways? Did she even
want to go (or (previously)
see reason to go) sniffing into the areas where the Fomor were forced, millenia ago? I suspect the answer to all is, "no."
So yeah: Mab, moving in power & with backup on-hand, opposed by
maybe (but probably not even) a random scout, or whatever? Sure, that happened & makes sense...
Without it implying Mab having any her usual one-step-ahead (or several srwps, for that matter) foresightedness.
... So I suspect the Fomor have the home field advantage, but various powers did know SOMETHING was up with the Fomor. They just didn't know what...
In addition to their "home field advantage" under water, the Fomor had another advantage: everyone already knew they were "making a move." They were taking advantage of the power-vacuum and WC weakness &c to make a bid to become more active in the world.
So troop-movements &c might be "expected" from them.
But a direct challenge to
all the Accorded powers at once? Making a simultaneous power-play against Mab
and Odin
and Ferrovax??!?
Who TF does
that?